This invention relates to the field of radomes, and particularly to radomes used at high temperatures.
The surfaces of high speed missiles are subjected to aerodynamic heating and to significant mechanical stresses and erosion. Consequently, radomes for high speed missiles must have both good high temperature strength and suitable dielectric properties within the entire temperature range at which the missiles operate.
Silica (SiO.sub.2) has proven useful for making high temperature radomes. However, there exists a continuing need for radome materials having greater high temperature strength and erosion resistance together with good dielectric properties.
Hot-pressed silicon nitride (Si.sub.3 N.sub.4) ceramics have been developed which have excellent high temperature (over 1000.degree. C.) strength and erosion resistance. Although pure Si.sub.3 N.sub.4 has adequate dielectric constants for radomes at room temperature and at elevated temperatures, when fabricated into components by standard ceramic production methods (using sintering aids, milling media, etc.), the dielectric losses are substantially increased, particularly at high temperatures.
The millimeter wave dielectric constants of prior art (hot-pressed, or reaction bonded) Si.sub.3 N.sub.4 materials are relatively high, being in the range of 7.5 to 9.5. These values imply that absolute tolerances in thicknesses need to be better than 0.001 inch in second order radomes (N=2, t.about.0.122 inch at 35 GHz). Additionally, they cause the power transmission and phase shift through the radome wall to be strongly dependent on the incident angle, so that matching to the antenna system cannot be readily achieved over wide angular ranges. This, in turn, introduces excessive reflective power loss as well as boresight error in a scanning radar system. Because the dielectric constant and dielectric loss change with temperature, matching wall thickness to the antenna is altered as the radome heats up under aerodynamic heating.